(1) Field of the Invention
The present invention relates to a diffractive optical element that can be used in a variety of applications, such as in optical information processing equipment and optical communication equipment. In particular, the invention relates to a diffractive optical element that polarizes light.
(2) Related Art
FIG. 1 is a cross-sectional drawing showing the construction of a conventional diffractive optical element 200.
The arrows in FIG. 1 show the courses taken by light rays. This is also the case in the following drawings.
As shown in FIG. 1, the diffractive optical element 200 is composed of a substrate 201 that is made of transparent plate glass and has a diffractive optical element pattern 203 formed on a main surface 202.
The incident light L0 is monochromatic and has the wavelength xcex. When this light strikes the diffractive optical element pattern 203 on the main surface 202 at an angle of 90xc2x0, the light will be diffracted as it passes the diffractive optical element pattern 203, producing zero-order diffracted light L1, and positive first-order diffracted light L2 and a negative first-order diffracted light L3 that each form the diffraction angle xcex81 with the light L1.
When it is assumed that the refractive index of the substrate 201 is n (where n greater than 1) and the pattern pitch of the diffractive optical element pattern 203 is xcex9, the value of the diffraction angle xcex81 can be found easily according to Equation 1 below.
xcex81=Sinxe2x88x921{(xcex/n)/xcex9}xe2x80x83xe2x80x83Equation 1
The diffraction angle xcex81 found in this way will usually be below the critical angle for total internal reflection by a surface (to be precise, a boundary face of a main surface) of the substrate 201. As a result, the zero-order light L1, the positive first-order light L2, and negative first-order light L3 all pass through the substrate 201 and exit a main surface 204 on an opposite side to the main surface 202.
Polarization is known to occur when the pattern pitch xcex9 is reduced for the diffractive optical element pattern 203 of this kind of diffractive optical element 200. There are high hopes that this property will enable new kinds of optical elements to be realized.
As Equation 1 clearly shows, reducing the pattern pitch xcex9 of the diffractive optical element pattern 203 to a value that is equal to or smaller than the wavelength xcex of the incident light will result in an increase in the diffraction angle. This gives rise to the problem of the diffracted light being trapped within the substrate 201.
FIG. 2 shows what happens in this case. A diffractive optical element pattern 303 having a pitch that is no greater than the wavelength of the incident light L0 is formed on a main surface 302 of a diffractive optical element 300. Diffraction caused by the diffractive optical element pattern 303 produces positive first-order diffracted light L2 and negative first-order diffracted light L3 that each form a larger diffraction angle xcex82 than the diffraction angle xcex81 in the case shown in FIG. 1. This diffraction angle xcex82 satisfies the condition for total internal reflection by main surface 304, so that the diffracted beams are completely reflected back into substrate 301.
Total internal reflection occurs whenever these reflected beams reach a main surface of the substrate 301, so that the diffracted light ends up being trapped within the substrate 301.
The following describes the case shown in FIG. 3 where a diffractive optical element pattern 213 is formed on a main surface 214 on an opposite side of a substrate 210 to a main surface 212 that is incident to the incident light L0. In FIG. 3, the pattern pitch xcex9 of the diffractive optical element pattern 213 is greater than the wavelength xcex of the light L0. In this case, diffraction by the diffractive optical element pattern 213 produces zero-order diffracted light L1, as well positive first-order diffracted light L2 and negative first-order diffracted light L3 that each form a diffraction angle xcex83 with the light L1. This diffraction angle xcex83 can be found using Equation 2 below.
xcex83=Sinxe2x88x921{(xcex/xcex9)}xe2x80x83xe2x80x83Equation 2
As shown in FIG. 3, the zero-order diffracted light L1, the positive first-order diffracted light L2 and the negative first-order diffracted light L3 each pass through the main surface 214 and out of the substrate 211.
However, when a diffractive optical element pattern is formed in this way, there is still the problem of the diffracted light being trapped in the substrate when the pattern pitch xcex9 of the diffractive optical element pattern is smaller than the wavelength xcex of the incident light.
One example of this case is a diffractive optical element 310 shown in FIG. 4. Diffraction occurs for the incident light L0 that strikes diffractive optical element pattern 313 formed on a main surface 314 of a substrate 311 to produce the zero-order diffracted light L1, the positive first-order diffracted light L2 and the negative first-order diffracted light L3. While the zero-order diffracted light L1 exits the main surface 314, the positive first-order diffracted light L2 and the negative first-order diffracted light L3 do not satisfy the condition for transmittive diffraction, and so are reflected back at a diffraction angle xcex84 that is found by Equation 3 below.
xcex84=Sinxe2x88x921{(xcex/n)/xcex9}xe2x80x83xe2x80x83Equation 3
These diffracted beams are hereafter subjected to total internal reflection by the main surfaces 312 and 314 and so end up being trapped within the substrate 310.
The above problem means that even if a diffractive optical element pattern is capable of polarizing light, the diffracted beams produced by the diffractive optical element pattern will not exit the substrate. This greatly limits the potential of such substrates as optical elements.
An optical pickup provided in a magneto-optical (MO) disk device reads the information stored on an MO disk by shining a laser beam at an information recording surface of the disk and splitting the light reflected off this surface using a polarizing beam splitter (hereinafter, xe2x80x9cPBSxe2x80x9d) in the form of a prism. While doing so, the pickup also obtains servo signals, such as the focus error signal and tracking error signal. A prism-shaped PBS is a relatively large component, and so makes miniaturization of the optical pickup difficult.
The present invention was conceived in view of the stated problems and has a first object of providing a diffractive optical element where light that has been diffracted by a diffractive optical element pattern exits the diffractive optical element even when a pattern pitch of the diffractive optical element pattern is equal to or smaller than the wavelength of the incident light.
The second object of the present invention is to provide a miniaturized optical pickup that uses a diffractive optical element as a polarizing beam splitter.
The first object of the present invention can be achieved by a diffractive optical element that diffracts incident light, including: a substrate with a first main surface and a second main surface, a refractive index of the substrate being equal to n where n is a value greater than one; a first diffractive optical element pattern that is formed on part of the first main surface with a pattern pitch xcex9 such that xcex/n less than xcex9xe2x89xa6xcex, where xcex is a wavelength of the incident light; and a second diffractive optical element pattern that is formed on one of the first main surface and the second main surface at a predetermined position on an optical path that diffracted light produced by the first diffractive optical element pattern takes within the substrate.
With the above construction, light is incident on the first diffractive optical element pattern that has a pattern pitch that is smaller than the wavelength of the incident light. Even if the light produced by transmittive diffraction or reflective diffraction has a large diffraction angle that satisfies the conditions for total internal reflection at a boundary face of a main surface of the substrate, this diffracted light will be further diffracted by the second diffractive optical element pattern. This lowers the angle at which the diffracted light is incident upon on the boundary face, so that the diffracted light can exit the substrate.
Here, a reflective film may be provided on a different main surface to the second diffractive optical element pattern at a predetermined position, the reflective film reflecting diffracted light produced by the second diffractive optical element pattern so that the diffracted light passes back through the substrate and then exits the substrate.
With the stated construction, the diffracted light can exit the substrate through the main surface opposite the main surface on which the reflective film is provided.
Here, the pattern pitch xcex9xe2x80x2 of the second diffractive optical element pattern may equal to the pattern pitch xcex9 of the first diffractive optical element pattern.
With the stated construction, the diffracted light produced by the first diffractive optical element pattern can exit the substrate in a same direction as an optical axis of the incident light.
Here, the second diffractive optical element pattern may be composed of slits, each slit having a slanted part when viewed in a cross-section taken in a plane that includes a main optical axis of the incident light and a main optical axis of the diffracted light, the diffracted light being incident on the slanted parts of the slits in the second diffractive optical element pattern.
With the stated construction, the diffracted light produced by the first diffractive optical element pattern will be incident on the slanted parts of each slit in the second diffractive optical element pattern. As the incident angle at the slanted parts is reduced, this angle can be kept below the critical angle. This results in transmittive diffraction of the incident light by the second diffractive optical element pattern, so that the diffracted light can exit the substrate.
Here, wherein the second diffractive optical element pattern may be formed of a plurality of slits that are curved in a plane that is parallel to the main surface on which the second diffractive optical element pattern is formed.
With the stated construction, the second diffractive optical element pattern has a lens effect. This means that the second diffractive optical element pattern can control the diffracted light to produce a parallel beam, a convergent beam or divergent beam which then exits the substrate.
The second object of the present invention can be achieved by an optical pickup that optically reads information that has been recorded on an optical recording medium, including: a laser beam exposing unit, including a light source that emits a laser beam, for focusing the laser beam on an information recording surface of the optical recording medium; a first polarizing beam splitter for splitting light reflected back off the information recording surface into first polarized light and second polarized light that is polarized in a different direction to the first polarized light; and a photoelectric conversion unit for receiving the first polarized light and the second polarized light and converting the first polarized light and the second polarized light into electrical signals, wherein the first polarizing beam splitter includes: a first substrate with a first main surface and a second main surface, a refractive index of the substrate being equal to n where n is a value greater than one; a first diffractive optical element pattern that is formed on part of the first main surface with a pattern pitch xcex9 such that xcex/n less than xcex9xe2x89xa6xcex, where xcex is a wavelength of the reflected light; and a second diffractive optical element pattern that is formed on one of the first main surface and the second main surface at a predetermined position on an optical path that diffracted light produced by the first diffractive optical element pattern takes within the first substrate.
With the stated construction, an optical pickup can be produced with the relatively small diffractive optical element of the present invention in place of the large polarizing beam splitting prism that was conventionally used. Such optical pickup is well suited to miniaturization.